1. Field of the Invention
The present invention relates to a method for manufacturing a billet using aqueous salt solutions, in particular, a method for manufacturing a billet, in which metal powders are mechanically deformed through an extruding or a rolling process, thereby preparing metal fillers which confer conductivity to paints, pastes, and plastics, and metal catalyst or electrode materials, sound absorption plate, and filter, which require large contacting area.
2. Description of the Prior Art
Conventionally, conductive paints and plastics used for screening electromagnetic waves have been prepared by mixing paints or resin with conductive fillers, in which the fillers have used metal powders, metal flakes, metal fibers, and metal-coated glass fibers. However, conductivity depends on the degree of contact between fillers so that fillers in the form of fibers having most excellent inter-fiber contact have been widely used. Meanwhile, metals used as catalysts and electrodes are required to have larger specific surface area for increasing reaction rates. As such, the specific surface area is maximized by using catalysts of the fiber form.
In addition, the filters employed in special circumstances, such as high temperature, are prepared by use of metal fibers, instead of synthetic fibers or natural pulps. Accordingly, various processes have been developed, for preparing metals in the form of thin fiber.
The metal fibers should have a diameter as small as possible, that is, 50 xcexcm or less, so that they can be used as conductive fillers. The smaller the diameter, the lower the amount of filler needed to be mixed with resin or paints. Hence, general wire-processing methods, such as a drawing method, cannot prepare such metal fibers. So, specific methods as follows have been employed.
Methods of preparing metal fibers for conductive fillers are classified into a bundle drawing, a vibration cutting, a melt spinning in-rotating water and so on.
A bundle drawing has advantages that fibers can be prepared to a diameter of 10 xcexcm or less, and length of fibers can be freely adjusted through the subsequent cutting procedure, but it suffers from the disadvantage of high cost in the procedures, such as a bundling of wires, repeated drawing, and a separating of wires after final drawing.
Additionally, a vibration cutting is advantageous in terms of simple processes, and applicability to all materials. However, vibration cutting has disadvantage that metal fibers with a diameter of 50 xcexcm or less are difficult to prepare. For instance, in order to obtain sufficient conductivity, fibers with a diameter of 10 xcexcm, prepared by a bundle drawing, are added in the amount of only 5 wt % to plastics, while fibers prepared by a vibration cutting are added in the amount of 35 wt % or more.
Though a melt spinning in-rotating water is more economical than said two prior processes, its products are limited to a diameter of 30 xcexcm or more, attributable to surface tension of the ejecting melt stream. Therefore, conventional metal fiber-preparing methods have drawbacks of limited diameter of fibers or high preparation cost.
Optimal conductive fillers for conductive plastics, catalyst and electrodes are used in lengths of 1000-20,000 xcexcm and a diameter of about 10-20 xcexcm; and for conductive paints in 10-20 xcexcm lengths of about 5 xcexcm in diameter.
Korean Pat. No. 092100, by the present inventors, refers to a method for manufacturing metal fibers, in which metal powders and salt powders in predetermined sizes are kneaded, filled in a mold, compression-molded, and processed into a billet, which is then extruded, and salts present in extruded materials are dissolved in water and thus removed. Thereby, metal fibers with a diameter of 20 xcexcm or less can be more easily prepared, compared with other methods, such as a bundle drawing.
In a method of said patent, metal powders are kneaded with salt powders, filled in a mold of suitable size, and compression-molded to obtain an extruding billet. The reason that metal powders are kneaded with salt powders is that metal powders are prevented from self-aggregating by salt powders. However, parts of metal powders tend to agglomerate, even though kneading is performed well. Such phenomenon have a negative influence on uniform control of diameter of metal fibers to be manufactured, thus a difficulty of quality control arises.
Accordingly, an object of the present invention for alleviating the problems as described above is to provide a method for manufacturing a powder-extruding billet in which most of metal powders are not agglomerated by salts, whereby the diameter of fibers prepared through the succeeding mechanical deformation processes, such as extruding or rolling, can be easily controlled.
Another object of the present invention is to provide a method for manufacturing a billet using aqueous salt solutions, in which, because of using aqueous salt solution, cut metal wires and mesh made from the wires, which are difficult to knead with salt powders, can be used as raw materials of metal fibers, whereby a selection range of raw materials can be broadened.
According to the present invention, there is provided a method for manufacturing a billet using aqueous salt solutions, comprising infiltrating aqueous salt solutions into metal powders or cut metal wires by adding the solutions to metal powders or cut metal wires filled in a cylindrical airtight container of desired diameter and height; evaporating water in aqueous salt solutions by heating said container containing a mixture of aqueous salt solutions infiltrated into the metal powders or metal wires; and separating dehydrated mixture of metal powders or metal wires and salts from the container, to obtain a billet.
In addition, there is provided a method for manufacturing a billet using aqueous salt solutions, comprising coating aqueous salt solutions to surfaces of metal wires or mesh made from the wires by settling metal wires or mesh into aqueous salt solutions; removing water in aqueous salt solutions by drying aqueous salt solutions-coated metal wires or mesh; and compression-molding said wires or mesh rolled into a cylindrical form of a suitable size, in a mold, to obtain a billet.
Metal fibers prepared according to the present invention are not limited in their kinds. In this regard, materials of metal powders, metal wires or mesh made from the wires are exemplified by metal composite obtained by plating Ni, Ag, Cu, Au or Pt, onto Pt, Pd, Al and Al alloy, Ag and Ag alloy, Ni and Ni alloy, Cu and Cu alloy, Ti and Ti alloy, Co and Co alloy, Fe and Fe alloy, and powders or wires thereof; and stainless steel.
Said salts can be selected from the group consisting of chloride salts, including sodium chloride, barium chloride, and potassium chloride; sulfates, including potassium sulfate, sodium sulfate, magnesium sulfate, and lithium sulfate; carbonates, such as potassium carbonate; phosphates, such as potassium phosphate; and fluoride salts, such as sodium fluoride.
Preferably, the aqueous salt solutions are saturated.
During manufacturing of said billet, before the mixture is separated from the container following the removal of water, the procedure in which aqueous salt solutions are again introduced and then evaporated is repeated several times. Thereby, a mixture ratio of metal and salts can be controlled. As necessary, when said materials are filled and evaporated, the mixture may be subjected to pressing under a proper pressure or vacuum. Also, it is favorable that said container is airtight.
The metal powders mentioned in the present invention, are referred to ones containing single metal or alloy powder thereof. In addition, the method for manufacturing the powders has no limitation.
The aqueous salt solutions means that water-soluble salts, such as sodium chloride, potassium sulfate, sodium sulfate, magnesium sulfate, potassium carbonate, potassium phosphate, and lithium sulfate, are dissolved to saturation in water in a temperature range of from room temperature to suitably high temperatures, determined by kinds of salts.
As a drawing process, an extrusion is used in the present invention, but a rolling may be used.
A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.